Optical port is audio only. It outputs Digital 5.1 sound if the channel carries it, which is usually HD channels only on certain programs.
HDMI eliminates one of the digital to analog to digital conversions, BUT it introduces HDCP copy protection and other control signaling which doesn't always work reliably in some equipment combinations. So HDMI should be slightly better in quality, but the hassle it can create may out weight it's benefits.
Just to clarify (assuming SA 8300 family converter): Like MacLeech said, The optical port is for digital audio only. However, it will work for audio on all channels not just the HD ones. I am running HDMI to my TV and I have had mixed results. I like the way the TV communicates with the box and automatically resizes the picture automatically. However sometimes it seems buggy and the TV will lock up when switching between SD and HD channels. I am running HDMI however I have an older receiver that has optical but not HDMI, so I am using the HDMI for picture up to the TV and optical for the audio to the receiver. I have a Logitech Harmony that keeps everything straight when switching sources.